Ionomeric resins are copolymers containing non-ionic repeating units and smaller amounts of ion containing units. Various ionomeric resins and blends thereof (sold by E. I. DuPont de Nemours & Company under the trademark Surlyn®, and by the Exxon Corporation under the trademarks Escor® and Iotek®, among others) have been used as golf ball covers. Ionomer covers are generally more durable than the traditional “balata” (trans polyisoprene, natural, or synthetic) rubber covers. The softer balata covers, although exhibiting enhanced playability properties, lack the durability properties required for repetitive play.
Ionomeric resins are generally ionic copolymers of an olefin, such as ethylene, and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid. In some instances, an additional softening monomer, such as an acrylate, can also be included to form a terpolymer. The pendent ionic groups in the ionomeric resins interact to form ion-rich aggregates contained in a non-polar polymer matrix. The metal ions, such as sodium, zinc, magnesium, lithium, potassium, calcium, nickel, manganese, copper, titanium, aluminum etc. are used to neutralize some portion of the acid groups in the copolymer, resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e. improved durability, etc.
The ionomeric resins utilized to produce cover compositions can be formulated according to known procedures such as those set forth in U.S. Pat. No. 3,421,766 or British patent no. 963,380, with neutralization effected according to procedures disclosed in Canadian patent nos. 674,595 and 713,631, wherein the ionomer is produced by copolymerizing the olefin and carboxylic acid to produce a copolymer having the acid units randomly distributed along the polymer chain. Ionic copolymers generally comprise one or more α-olefins, and from about 9 to about 20 weight percent of α,β-ethylenically unsaturated mono- or dicarboxylic acid. The base copolymer is neutralized with metal ions to the extent desired.
The ionomeric resins utilized in the golf ball industry are generally copolymers of ethylene with acrylic (e.g. Escor®) and/or methacrylic (e.g. Surlyn®) acid. In addition, two or more types of ionomeric resins may be blended into the cover compositions in order to produce the desired properties of the resulting golf balls. Generally, ionomers in the mid-acid range have lower flexural modulus than the high-acid ionomers, and high acid ionomers also tend to be more brittle or less durable than mid-acid ionomers. However, the mid-acid ionomers are generally less expensive. Higher flexural modulus materials when used as an inner cover layer or outer cover layer reduce spin and increase coefficient of restitution at high swing speeds. Hence, there remains a need to modify mid-acid ionomers to increase flexural modulus.